In the manufacture of permanent magnet materials, sintering and casting processes have been known for a long time. In recent years it has become common practice to mix powdered magnetic materials with a plastic binder to form a mixture which is formed into the desired shape by pressing, injection molding and extruding and rolling, with or without a magnetic directional field for aligning the particles when they are anisotropic. It has also been suggested to use metals with an appropriate melting point as the binder instead of plastic material.
The use of duroplastic and thermoplastic synthetic materials as the plastic binder can cause difficulties, particularly if the permanent magnet material forming the particles of the bonded magnet have a great affinity for oxygen. For instance, finely powdered iron, bismuth-manganese, SE-magnets and cobalt, rare-earth magnetic powder material have high affinity for oxygen and their effectiveness can be reduced by oxidation. Binding these materials with plastic offers substantial technological advantages; however, the plastic binder and the normal procedure for processing plastic bonded magnets has not been generally successful, from a commercial standpoint because it was not economical or practical to prevent undue oxidation of the particles. One has, therefore, essentially restricted the production of plastic bonded permanent magnetic molded bodies to mixtures using permanent magnet materials which are resistant to oxidation. In the processing of permanent magnet materials with great affinity for oxygen, corrosion cannot be prevented with certainty with the customary plastic binding agents or normal methods of making bonded magnets. There are, however, situations in which the application of such oxygen affinitive materials would offer particular advantages.